1. Field of the Invention
The present invention relates to a near-field light generating device for use in heat-assisted magnetic recording where a magnetic recording medium is irradiated with near-field light to lower the coercivity of the magnetic recording medium for data recording, and a heat-assisted magnetic recording head, a head gimbal assembly, and a magnetic recording device each of which includes the near-field light generating device.
2. Description of the Related Art
Recently, magnetic recording devices such as a magnetic disk drive have been improved in recording density, and thin-film magnetic heads and magnetic recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a reproducing head including a magnetoresistive element (hereinafter, also referred to as MR element) intended for reading and a recording head including an induction-type electromagnetic transducer intended for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
Magnetic recording media are discrete media each made of an aggregate of magnetic fine particles, each magnetic fine particle forming a single-domain structure. A single recording bit of a magnetic recording medium is composed of a plurality of magnetic fine particles. For improved recording density, it is necessary to reduce asperities at the borders between adjoining recording bits. To achieve this, the magnetic fine particles must be made smaller. However, making the magnetic fine particles smaller causes the problem that the thermal stability of magnetization of the magnetic fine particles decreases with decreasing volume of the magnetic fine particles. An index to show the thermal stability of magnetization of magnetic fine particles can be expressed as KuV/kBT, for example. Ku is the anisotropic energy of the magnetic fine particles, V is the volume of a single magnetic fine particle, kB is the Boltzmann constant, and T is the absolute temperature. The greater KuV/kBT, the higher the thermal stability of magnetization of the magnetic fine particles. Making the magnetic fine particles smaller translates into smaller V, which by itself shrinks KuV/kBT. Then, Ku may be increased instead. However, higher Ku leads to higher coercivity of the magnetic recording medium. Since the magnitude of the magnetic field to be produced by the magnetic head for recording is mostly determined by the saturation flux density of the soft magnetic material that forms the magnetic pole, there is essentially an upper limit to the coercivity of the magnetic recording medium at which data recording is possible.
To solve the foregoing problem regarding the thermal stability, there has been proposed a technology called heat-assisted magnetic recording. Heat-assisted magnetic recording uses a magnetic recording medium made of high-Ku magnetic material, and applies a magnetic field and heat to the magnetic recording medium at the same time to lower the coercivity of the magnetic recording medium for data recording. Hereinafter, a magnetic head for use in heat-assisted magnetic recording will be referred to as a heat-assisted magnetic recording head.
In heat-assisted magnetic recording, near-field light is typically used as a means for applying heat to the magnetic recording medium. A commonly known method for generating near-field light is to use a near-field optical probe or so-called plasmon antenna, which is a piece of metal that generates near-field light from plasmons excited by irradiation with laser light. For example, U.S. Pat. No. 6,768,556 discloses a near-field optical probe (plasmon antenna) which includes a metal scatterer in the shape of a circular cone or the like formed on a substrate, and a film of a dielectric or the like formed around the scatterer.
JP-A 2008-111845 discloses such a technique that an apex of a scatterer that generates near-field light when irradiated with laser light is brought close to a magnetic recording medium, and electric charges are concentrated on this apex so that near-field light of high intensity occurs in the vicinity of the apex.
Conventional typical plasmon antennas generate near-field light when directly irradiated with laser light. If such a plasmon antenna is used as a near-field light generating part to achieve heat-assisted magnetic recording, however, there arises the following problem.
That is, while a plasmon antenna converts the laser light applied to itself into near-field light as mentioned above, its light use efficiency is known to reach only about 10% at most. Some 90% of the energy of the laser light applied to the plasmon antenna is reflected by the surface of the plasmon antenna, or converted into thermal energy and absorbed by the plasmon antenna. The plasmon antenna is small in volume since the size of the plasmon antenna is set to be smaller than or equal to the wavelength of the light. The plasmon antenna therefore shows a significant increase in temperature when it absorbs the thermal energy. For example, a simulation has shown that a plasmon antenna made of Au, having the shape of an equilateral triangular plate with each side of 300 nm and being 50 nm thick reaches a temperature of 500° C. when it absorbs 17-mW laser light at room temperatures.
Such a temperature increase makes the plasmon antenna expand in volume and protrude from a medium facing surface, which is the surface of the heat-assisted magnetic recording head to face the magnetic recording medium. This causes an end of the reproducing head located in the medium facing surface to get farther from the magnetic recording medium, thereby causing the problem that a servo signal cannot be read during recording operations.
Under the circumstances, the inventors of the present application have devised such a technique that laser light propagating through a waveguide is coupled with a surface plasmon generating element in a surface plasmon mode via a buffer part, instead of directly irradiating a plasmon antenna with the laser light, and surface plasmons excited on the surface plasmon generating element are then allowed to propagate to the medium facing surface to obtain near-field light. This technique can avoid an excessive temperature increase of the surface plasmon generating element since the surface plasmon generating element is not directly irradiated with the laser light. Furthermore, according to this technique, one of the surfaces of the surface plasmon generating element farther from the magnetic pole can function as the surface with which the laser light is to be coupled via the buffer part (this surface is hereinafter referred to as coupling surface). This makes it possible to prevent the laser light from being absorbed by the magnetic pole.
How to guide laser light to the surface plasmon generating element in a heat-assisted magnetic recording head will now be considered. Heat-assisted magnetic recording typically uses a laser diode as the means for generating laser light. U.S. Patent Application Publication No. 2006/0187564 A1 discloses a technique in which a laser diode is placed on a side of the slider farther from the medium facing surface, and laser light emitted from this laser diode is guided to the medium facing surface through a wave guide formed in the slider. For the location of the laser diode in a heat-assisted magnetic recording head, the one disclosed in U.S. Patent Application Publication No. 2006/0187564 A1 is considered to be ideal because it facilitates heat dissipation of the laser diode and fabrication of the head, and allows stable guiding of the laser light to the medium facing surface. Thus, placing a laser diode on the side of the slider farther from the medium facing surface is conceivable also for a heat-assisted magnetic recording head that employs the foregoing technique of coupling laser light with the surface plasmon generating element via the buffer part.
Reference is now made to FIG. 35 and FIG. 36 to describe the relationship between the coupling surface of the surface plasmon generating element mentioned above and the direction of polarization of laser light that propagates through the waveguide. Each of FIG. 35 and FIG. 36 shows a configuration where a surface plasmon generating element 301 is laid over a waveguide with a buffer part in between. Hereinafter, laser light that propagates through a waveguide in such a stack structure including the waveguide will be referred to as TE-polarized light if its electric field oscillates in a direction parallel to the top and bottom surfaces (the surfaces opposite to each other in the stacking direction) of the waveguide, and will be referred to as TM-polarized light if its electric field oscillates in a direction perpendicular to the top and bottom surfaces of the waveguide. FIG. 35 shows the mode in which TE-polarized light propagates through the waveguide. In this mode, the electric field of the laser light propagating through the waveguide oscillates in a direction parallel to the coupling surface 301a of the surface plasmon generating element 301. FIG. 36 shows the mode in which TM-polarized light propagates through the waveguide. In this mode, the electric field of the laser light propagating through the waveguide oscillates in a direction perpendicular to the coupling surface 301a of the surface plasmon generating element 301. When the two modes shown in FIG. 35 and FIG. 36 are compared as to the intensity of surface plasmons occurring on the surface plasmon generating element 301, the mode shown in FIG. 36 provides surface plasmons of far higher intensity than the mode shown in FIG. 35 does. A simulation was performed with the waveguide of the same shape and material, the surface plasmon generating element 301 of the same shape and material, and the laser light of the same wavelength, and the results showed that the intensity of the surface plasmons occurring in the mode shown in FIG. 36 was approximately 50 times that of the surface plasmons occurring in the mode shown in FIG. 35. The mode shown in FIG. 36 is thus obviously preferred.
On the other hand, laser diodes include one that emits TE mode light whose electric field oscillates in a direction parallel to an active layer which is the layer for emitting the laser light (such a laser diode will be hereinafter referred to as a TE-polarization laser), and one that emits TM mode light whose electric field oscillates in a direction perpendicular to the active layer (such a laser diode will be hereinafter referred to as a TM-polarization laser). Of these, the TE-polarization laser is more common.
When forming a heat-assisted magnetic recording head in which laser light emitted from the laser diode and propagating through the waveguide is to be coupled with the surface plasmon generating element via the buffer part, there arises the following problem. To form such a head, the waveguide, the buffer part and the surface plasmon generating element may be stacked in this order or in the reverse order on the top surface of a base. In addition, the laser diode may be mounted on the top surface of a support member, and a side surface of the support member may be joined to a surface of the slider farther from the medium facing surface. In this case, the top surface of the base and the top surface of the support member become parallel to each other. FIG. 37 shows the positional relationship among the surface plasmon generating element 301, the waveguide 302, the laser diode 303 and the support member 304 in a head formed in the above-described manner. If a typical TE-polarization laser is used as the laser diode 303 in this configuration, it results in the mode where TE-polarized light, i.e., laser light whose electric field oscillates in the direction parallel to the coupling surface 301a of the surface plasmon generating element 301, propagates through the waveguide 302 (the mode shown in FIG. 35) as shown in FIG. 37. This causes the problem that the surface plasmon generating element 301 cannot generate surface plasmons of high intensity.